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1 Overview of Overhead Travelling Cranes
An overhead travelling crane is a material handling hoisting equipment mounted across workshops, warehouses and stockyards. Supported by tall concrete columns or metal frames on both ends, it features a bridge-like structure. Its bridge frame travels longitudinally along rails installed on elevated supports on both sides. This design fully utilizes the space beneath the bridge for material handling without obstruction from ground facilities. It is the most widely used and largest-in-quantity type of hoisting machinery.
A conventional overhead travelling crane mainly consists of a hoisting trolley, bridge travelling mechanism and metal bridge structure. The hoisting trolley is further composed of a hoisting mechanism, a trolley travelling mechanism and a trolley frame.
The hoisting mechanism includes an electric motor, brake, reducer, drum and pulley block. The motor drives the drum to rotate via the reducer, winding or unwinding the steel wire rope to lift and lower loads. The trolley frame is a welded structure that supports and installs the hoisting mechanism, trolley travelling mechanism and other components.
The driving modes of the crane travelling mechanism fall into two categories. The first is central drive, where one motor drives the driving wheels on both sides through a long transmission shaft. The second is individual drive, with a separate motor for each set of driving wheels. Medium and small-sized overhead travelling cranes commonly adopt the three-in-one integrated drive unit combining motor, reducer and brake. For heavy-duty models, universal couplings are widely used in driving assemblies to facilitate installation and adjustment.
Generally, the travelling mechanism is equipped with four driving and driven wheels. For extra-heavy loads, additional wheels are fitted to reduce wheel load. When more than four wheels are used, articulated equalizing frames must be installed to evenly distribute the load across all wheels.
The metal bridge structure is made up of main girders and end girders, classified into single-girder and double-girder types. A single-girder bridge comprises one main girder and end girders at both ends of the span, while a double-girder bridge has two main girders paired with end girders.
Main girders and end girders are rigidly connected. Wheels are mounted at both ends of end girders to support the bridge moving along elevated rails. Rails are welded on main girders for the travel of the hoisting trolley. There are various structural types for main girders, among which box girder, four-truss and open-web truss structures are typical.
Box girders are divided into standard rail double box girders, offset rail double box girders and offset rail single box girders. The standard rail double box girder is a mainstream basic type. Its main girder is assembled with top and bottom flanges and vertical webs on both sides, with trolley rails arranged along the centerline of the top flange. It features simple structure, easy fabrication and suitability for mass production, yet has relatively heavy self-weight.
The cross-section of offset rail double box girders and offset rail single box girders consists of top and bottom flanges as well as main and auxiliary webs with unequal thickness. Trolley rails are placed above the main web, and short stiffeners inside the box can be omitted. The offset rail single box girder replaces two separate girders with one wide-flange box girder, which is lighter in weight but more complex to manufacture.
The four-truss structure is a closed spatial assembly of four plane trusses, usually fitted with walkway plates on the upper horizontal trusses. It boasts light weight and high rigidity, but has large overall dimensions, complicated fabrication and low fatigue strength, so it is rarely manufactured nowadays.
The open-web truss structure is similar to the offset rail box girder. It forms a closed structure with four steel plates. Except for the solid web I-shaped main web, the other three steel plates are cut with multiple openings as designed to form an open-web truss without diagonal rods. Walkway plates are laid on upper and lower horizontal trusses. The travelling mechanism and electrical equipment are installed inside the bridge. This type has light self-weight and high overall rigidity and is widely applied in China.
Conventional overhead travelling cranes are mostly electrically driven, operated either from a driver’s cabin or by remote control. Their rated lifting capacity can reach 500 tons and the maximum span up to 60 meters.
Metallurgical special overhead travelling cranes are applied to specific production procedures in the iron and steel industry. They have a basic structure similar to conventional cranes, but are equipped with special working mechanisms or devices on the hoisting trolley. Featuring frequent operation, harsh working conditions and high working class, they are mainly categorized into five types: ladle cranes, tong cranes, ingot stripping cranes, charging cranes and forging cranes.
During my internship, I worked with turning-over cranes, which vertically lift high-temperature steel billets and transfer them onto billet transport vehicles.
Overhead travelling cranes greatly improve production efficiency and convenience. However, mechanical defects, harsh on-site operating environments and frequent use often lead to malfunctions. This paper analyzes common mechanical faults of overhead travelling cranes concerning steel wire ropes, drums and rope clamps, reducer gears, brakes, wheels and rails. Corresponding preventive suggestions are put forward to avoid safety accidents and reduce maintenance frequency.
The stress condition of steel wire ropes during operation is complex. Wires at different positions (inner or outer layers) bear uneven loads even under simple tension. Besides tension, bending stress is generated when ropes wrap around drums and pulleys, together with extrusion force between adjacent wires. For this reason, static calculation is generally adopted instead of precise stress analysis.
The maximum static tension of the steel wire rope shall satisfy the formula below:
\(P_{max}\leq\frac{P_d}{n}\)
Where:
\(P_{max}\) — Maximum allowable static tension during operation
\(P_d\) — Breaking tension of the steel wire rope
n — Safety factor
\(P_{max}=\frac{Q+q}{a\eta}\)
Where:
Q — Rated lifting capacity of the crane
q — Weight of the hook block
a — Number of load-bearing rope branches of the pulley block
\(\eta\) — Overall efficiency of the pulley block
The maximum allowable working tension of the rope is calculated as:
\(P=\frac{P_d}{n}\)
Where:
P — Rated maximum static tension of the steel wire rope
The rope is deemed safe when \(P\geq P_{max}\).
Overload is the primary cause of rope breakage. In addition, repeated bending during wrapping around drums and pulleys accelerates wear and fracture. Other influencing factors include the diameter of drums and pulleys, working environment, operation type and daily maintenance.
Never exceed the rated lifting capacity during operation.
Select steel wire ropes matching the operation type and working environment.
Double-lay ropes are recommended. They are fabricated by first twisting wires into strands, then twisting strands around a core. Hemp cores are commonly used; asbestos cores or metal cores made of soft steel wires are suitable for high-temperature working scenarios. Rope cores shall be coated with lubricant before assembly to reduce friction and abrasion between wires. Double-lay ropes feature good flexibility and easy production, and are widely used in practice.
Perform regular lubrication on steel wire ropes.
Avoid sudden impact loads on the rope during operation.
The drum is a critical load-bearing component. Common faults include wall thinning, pitting and fracture, caused by continuous extrusion and friction between the drum and steel wire rope. A drum will rupture when its wall thickness decreases beyond the bearing limit. In accordance with national standards, the drum must be replaced in a timely manner when wall wear reaches 20% of the original thickness or cracks appear. Meanwhile, keep the operating environment clean and maintain lubrication for drums and steel wire ropes.
The reducer is a key transmission component of overhead travelling cranes. It adjusts the high rotating speed of the motor to the required speed via gear meshing while transmitting torque. Typical gear faults include tooth breakage, pitting, scuffing and surface wear, with root causes listed as follows:
Fatigue fracture caused by short-term overload, impact load or repeated bending.
Stress concentration raised by rough tooth surfaces and protrusions, or contaminated lubricant.
Lubrication failure due to excessive temperature.
Surface wear caused by hard particles entering friction surfaces.
Strictly prohibit overload operation. Start and brake the crane gently; sudden reverse running is forbidden under normal conditions.
Replace lubricant regularly, clean the reducer housing thoroughly and select lubricant of appropriate type.
Frequently inspect the cleanliness of lubricant and replace contaminated oil promptly.
The brake is an essential safety component of overhead travelling cranes, which prevents suspended loads from falling and realizes equipment stopping. A well-functioning brake guarantees operational accuracy and production safety. Two brake shoes are symmetrically installed on both sides of the brake wheel and connected through lever mechanisms and brake springs, ensuring synchronous contact and separation of the two shoes against the brake wheel.
When the mechanism powers off and stops running, the brake thruster cuts off power simultaneously and stops outputting thrust. The brake spring pushes the brake arms and shoes to press tightly against the brake wheel, generating rated braking torque to stop the equipment. When the mechanism is energized, the thruster operates and produces sufficient thrust to compress the brake spring, pushing the brake arms outward. The shoes separate from the brake wheel, releasing braking pressure and torque.
Holding function: Keep stationary loads suspended stably in the air for the hoisting mechanism.
Stopping function: Dissipate kinetic energy of moving parts and halt the mechanism within a specified time or travel distance.
Positioning function: After the mechanism reaches the preset position and electric braking completes, the brake resets and clamps the brake wheel to fix the crane in place.
The brake directly affects the operating accuracy and reliability of all mechanisms, and is closely related to personal and equipment safety. It shall be adjusted regularly in accordance with relevant standards.
Common faults between wheels and rails include rail gnawing, uneven trolley height and trolley slipping.
Rail gnawing severely shortens the service life of cranes and may lead to fatal accidents in severe cases. Main causes include installation errors, uneven friction, excessive wear of transmission parts and excessive clearance of key connections resulting in asynchronous braking. Cranes shall be installed and maintained by qualified organizations. Daily inspection and management shall be strengthened to identify root causes and eliminate rail gnawing.
Uneven trolley height is another major hazard. It may cause one trolley wheel to hang in the air or bear insufficient load, leading to severe vibration. This fault is mainly attributed to unqualified installation errors and uneven self-weight distribution of the trolley, which requires comprehensive troubleshooting and rectification.
Bright wear marks on rail sides, with burrs and iron scraps in severe cases.
Bright spots and burrs on the inner side of wheel flanges.
Obvious variation of the gap between wheel flanges and rail sides after short-distance travel.
Deviation and swinging of the crane body during starting or braking.
Abnormal friction noise or wheel climbing in serious rail gnawing conditions.
Trolley slipping is mainly caused by dirty rails, harsh starting, uneven trolley rails, out-of-round wheels and unbalanced wheel load. Timely inspection and troubleshooting are required to eliminate this problem.
Given the numerous components of overhead travelling cranes, maintenance and inspection are divided into weekly, monthly and annual cycles based on different technical requirements.
Perform inspection and maintenance once a week:
Check all nuts, cotter pins and positioning plates of brakes for completeness and tightness. Inspect levers and springs for cracks, and pins, bolts and buffer washers of brake wheels for looseness. Ensure reliable braking performance. The opening of brake shoes shall be less than 1.0 mm with equal gaps on both sides of the brake wheel, and all pins shall move flexibly without jamming.
Verify accurate positioning and flexible operation of safety protection switches and limit switches, especially the hoist limit switch.
Check the winding condition of steel wire ropes on drums and pulleys to prevent rope disengagement, crossing, knotting and twisting. Ensure all rope clamp bolts are fastened and double-nut anti-loosening devices are intact.
Inspect fastening screws on coupling sealing covers of the hoisting mechanism for looseness and missing parts.
Check the operation of all transmission parts for abnormal noise.
Confirm good lubrication of all lubrication points.
Remove foreign objects on rails that may hinder crane travel.
Complete all weekly inspection items plus the following contents on a monthly basis:
The wear of brake shoe liners shall not exceed 2 mm, and the contact area between liners and brake wheels shall be no less than 70%. Inspect the fixation, wear and lubrication of all pins. The wear of pins shall not exceed 5% of the original diameter, and the ovality shall be less than 0.5 mm.
Check steel wire ropes for wear and broken wires, and inspect lubrication status.
Examine lifting appliances for cracks. The wear of critical sections shall not exceed 5% of the original thickness. Ensure anti-loosening devices of hook nuts and all parts of the hook block are complete. The hook shall rotate flexibly without jamming.
Check all bolts for looseness and missing parts.
Inspect and retighten anchor bolts of motors, reducers and other equipment.
Check the oil level of reducers, which shall stay within the specified range. Take measures to eliminate oil leakage.
Lubricate gears.
Inspect rope wear at equalizer pulleys, and lubricate pulleys and pulley shafts.
Check pulleys for flexibility, damage and cracks, with special attention to fixed pulley shaft wear.
The surface unevenness of brake wheels shall not exceed 1.5 mm. Brake wheels shall be crack-free, with radial runout less than 0.3 mm.
Check couplings: keys and keyways shall be intact and tight. The axial play of transmission shafts between two couplings shall range from 2 mm to 7 mm.
Inspect the travel of main and auxiliary trolleys. Rail gnawing, three-point support and swinging during starting and stopping are prohibited. The wear of wheel flanges shall not exceed 50% of the original thickness, and wheel tread wear shall not exceed 3% of the original wheel diameter.
Inspect main rails: check bolts and clamps for looseness and missing parts, and rails for cracks. The gap between two rail joints shall be 1~2 mm in summer and 3~5 mm in winter. The vertical and horizontal misalignment at joints shall not exceed 1 mm.
Thoroughly clean dirt on the crane.
This thesis focuses on the turning-over overhead travelling crane applied in the steel production area. It conducts a detailed analysis on common mechanical faults of overhead travelling cranes, and puts forward corresponding preventive and treatment measures as well as relevant maintenance requirements. It is expected to reduce equipment maintenance frequency and create practical economic benefits through fault analysis and standardized maintenance.
[1] Safety Technology for Hoisting Machinery. Sichuan University Press
[2] General Mechanical Equipment. Zhao Quanchang. Metallurgical Industry Press
[3] Engineering Cranes. Gu Dimin. China Architecture & Building Press
[4] Mechanical Design Handbook (Volume II). Cheng Daxian. Chemical Industry Press
